The present invention relates to reforming, especially dehydrocyclizing, hydrocarbons to form aromatics using a large pore zeolite catalyst. Reforming embraces several reactions, such as dehydrogenation, isomerization, dehydroisomerization, cyclization and dehydrocyclization. In the process of the present invention, aromatics are formed from the feed hydrocarbons to the reforming reaction zone, and dehydrocyclization is believed to be the most important reaction in the present process.
U.S. Pat. No. 4,104,320, granted on Aug. 1, 1978, discloses that it is possible to dehydrocyclize paraffins to produce high octane aromatics with high selectivity using a monofunctional non-acidic large-pore zeolite catalyst. The catalyst consists essentially of a type-L zeolite having exchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium and contains at least one Group VIII noble metal (or tin or germanium). In particular, catalysts having platinum on potassium form L-zeolite exchanged with a rubidium or caesium salt were claimed to achieve exceptionally high selectivity for n-hexane conversion to benzene. As disclosed in U.S. Pat. No. 4,104,320, the L zeolites are typically synthesized in the potassium form. A portion, usually not more than 80%, of the potassium cations can be exchanged so that other cations replace the exchangeable potassium.
Results as in U.S. Pat. No. 4,104,320 were also reported by J.R. Bernard at the 5th International Zeolite Conference in 1980. But, while it was clear that the improvement in selectivity was significant, particularly for C.sub.6 -C.sub.8 paraffins and especially for C.sub.6 paraffins, it was independently found that the catalyst had limited commercial potential. At conventional low pressure reforming conditions (about 200 psig) catalyst life was measured in hours and days, obviously an unacceptably short cycle life. Nonetheless, it had now been demonstrated that a platinum-containing alkali metal exchanged L-zeolite catalyst could achieve exceptionally high selectivity for the conversion of paraffins to aromatics. Advancing that discovery to a commercial catalyst became a new goal of catalytic reforming research.
An important step forward was disclosed in U.S. Pat. No. 4,434,311, granted on Feb. 28, 1984; U.S. Pat. No. 4,435,283, granted on Mar. 6, 1984; U.S. Pat. No. 4,447,316, granted on May 8, 1984 and U.S. Pat. No. 4,517,306, granted on May 14, 1985. These patents describe catalysts comprising a large pore zeolite exchanged with an alkaline earth metal (barium, strontium, or calcium, preferably barium) containing one or more Group VIII metals (preferably platinum) and their use in reforming petroleum naphthas. An essential element in the catalyst is the alkaline earth metal. Especially when the alkaline earth metal is barium, and the large-pore zeolite is L-zeolite, the catalysts were found to provide even higher selectivities than the corresponding alkaliexchanged L-zeolite catalysts disclosed in U.S. Pat. No. 4,104,320. Moreover, another equally significant benefit achieved by the use of an alkaline earth metal exchanged L-zeolite catalyst is that the catalyst retained its activity over a commercially acceptable cycle life.
The discovery that alkaline earth metal exchanged large pore zeolite reforming catalysts, especially the barium exchanged L-zeolite containing platinum, provide high selectivity even relative to the corresponding alkali metal exchanged L-zeolite containing platinum was surprising. These catalysts are all substantially "non-acidic" and therefore have been referred to as "monofunctional catalysts".
Having discovered a selective catalyst with an acceptable cycle life, commercialization seemed straight-forward. Unfortunately, that was not the case. It was found that the high selectivity, large pore zeolite catalysts containing a Group VIII metal were unexpectedly susceptible to sulfur poisoning. U.S. Pat. No. 4,456,527 discloses this discovery. Specifically, it was found that the concentration of sulfur in the hydrocarbon feed should be at ultra-low levels, preferably less than 100 parts per billion (ppb), more preferably less than 50 ppb to achieve improved stability/activity for the catalyst used in the process.
After recognizing the sulfur sensitivity of these catalysts and determining the necessary level of sulfur control, commercialization again seemed feasible. However, as is sometimes the case with an emerging technology, another set back was encountered. It was found that certain of the large pore zeolite catalysts are surprisingly sensitive to the presence of water while under reaction conditions. Water greatly accelerates the rate of deactivation of some of these catalysts.
Water sensitivity is an extremely serious drawback. Water is produced at the beginning of each cycle when the catalyst is reduced with hydrogen. Water can also be produced during process upsets when water leaks into the reformer feed or the feed becomes contaminated with an oxygen-containing compound. If the catalyst must be protected from water, then expensive additional equipment is required.